The metal plating industry consumes enormous amounts of water for rinsing the plated materials. The consumption of water can be in the range of from 10 liters per minute up to 1200 liters per minute depending on the size of the process. The rinse water becomes contaminated during the plating process when the plated object is rinsed upon removal from the plating bath. The contaminated rinse water is toxic as it can contain several heavy metals such as chrome, copper, zinc, lead, nickel, iron and chemicals such as cyanide. Therefore it cannot be released to the environment without further treatment to remove the heavy metal ions and toxic compounds.
In conventional waste water treatment, heavy metal contaminated waste streams are treated with a complexing agent such as sodium, magnesium or calcium hydroxide to form a metal complex. The metal complex is then precipitated out of the waste stream and settles by gravity to the bottom of a holding tank. The waste water in the holding tank is then clarified by removing the precipitate.
The effluent, which is highly alkaline, neutralized by acid dosing. After this treatment process, the waste water stream can still contain up to 10 mg per liter of heavy metals which is an unacceptable concentration for release to the environment. In order to meet the environmental standards for this type of waste water discharge, which are in the range of 1 to 2 mg per liter for sewer discharge and 0.1 to 0.5 mg per liter for open water discharge, the waste stream must be further treated using a series of ion exchange columns to remove heavy metal ions. While the final concentrations of contaminants released to the environment may be within environmental guidelines, such releases do place additional strain on municipal sewage systems and will accumulate in the environment with detrimental long term effects on the biosphere. The precipitated sludge containing the concentrated heavy metal hydroxide is an extremely hazardous waste and must be disposed of using special facilities at great expense to industry.
The cost of buying water for metal plating industries ranges from $6000.00 to $10,000.00 per year for a process consuming 40 liters per minute. Therefore, there is a need to reduce this cost by recycling as much of the process water as possible.
There are a number of known heavy metal recovery systems. For example, U.S. Pat. No. 4,880,511, entitled "PROCESS AND APPARATUS FOR RECOVERY OF PRECIOUS METAL COMPOUND" issued to Sugita on Nov. 14, 1989, describes a process and apparatus for recovery of precious metal compounds such as gold. This process utilizes a reverse osmosis membrane to separate the contaminant from the waste stream. The contaminant is further concentrated using an electrodialysis process employing anion-exchange columns and cation-exchange columns. The use of a reverse osmosis membrane means that waste stream operating at higher pressures (typically in the range of 250 psi to 400 psi) and flow rates must be kept high to create turbulence otherwise membranes will quickly foul. Therefore, this process is not well suited to industrial plating industries due to a greater tendency towards membrane foiling because of higher pressures and increased power input. The waste stream is preferably processed at low pressures to reduce energy input and high velocity to reduce membrane foiling. U.S. Pat. No. 4,678,584 entitled "METHOD OF REMOVING HEAVY METAL FROM WASTEWATER STREAMS" issued to Elfine on Jul. 7, 1989, teaches a method for treating heavy metal-containing wastewater streams using sodium trithiocarbonate as a complexing agent and then precipitating the heavy metal complex out of the waste water. The resulting concentrated heavy metal sludge is further processed using standard metallurgical techniques to remove the metals in economically useful forms. This method still rests in a heavy metal sludge which requires further processing and disposal and therefore added operating expenses.